1. Field of the Invention
The present invention relates to a hydraulically damped mounting device.
2. Description of the Prior Art
In our published European Patent Application No. 0115417A we discussed hydraulically damped mounting devices in which two parts of a piece of machinery e.g. a car engine and chassis were connected together by a resilient wall which, together with a partition, defined a working chamber for hydraulic fluid. The working chamber was connected to a compensation chamber by a passageway (usually elongate), the compensation chamber being separated from the working chamber by a rigid partition. A flexible diaphragm was in direct contact with the liquid in the working chamber and separated that liquid from a gas pocket. The purpose of that mounting device was to provide a frequency-responsive device, which could isolate high and low frequency vibrations independent of their amplitude (within certain amplitude ranges).
In the hydraulically damped mounting device of EP-A-0115417 the two parts of the machinery were respectively connected to a cup and a boss with the deformable, preferably resilient, wall holding the boss within the mouth of the cup. However, hydraulically damped mounting devices do not all have this configuration, and a hydraulically damped mounting device is known from e.g. U.S. Pat. No. 3,642,268 in which the anchor part for one part of the vibrating machinery is in the form of a hollow sleeve with the other anchor part in the form of a rod or tube extending approximately centrally and coaxially of the sleeve. Resilient walls then join the sleeve and the tube and, in that patent specification, define two chambers connected by a passageway. The chambers are filled with hydraulic fluid and the movement of the fluid from one chamber to the other through the passageway damps the vibration of the parts of the machinery attached to the respective anchor points.
Mounting devices of this type, in which there is a first anchor part e.g. in the form of a tube, within a second anchor part in the form of a sleeve, with the sleeve and tube being connected by resilient walls, are desirable because they permit a compact construction, all parts being enclosed within the sleeve, but problems have been encountered. The main problem is that rubber, the material commonly used to form the resilient walls, should not be put under a tensile load, and any vibration which compresses the walls bounding one chamber also requires an expansion in the walls bounding the other chamber, i.e. tensile stresses are generated. In U.S. Pat. No. 3,642,268 an attempt is made to solve this problem, by forcing the resilient walls into the sleeve and/or expanding the tube to pre-compress the resilient walls. If all the walls are pre-compressed during the manufacture of the mounting device, then limited vibration is possible without exceeding the compression in the walls, which would generate tensile forces. However in practice, the strains placed on the walls due to vibration are 25% to 50% and it is difficult to pre-compress the walls sufficiently to permit this, as the rubber becomes unstable. Therefore, it has not proved possible to manufacture a commercially practicable device of this design.
Published European patent application No. 0009120 also discloses a hydraulically damped mounting device having an anchor part in the form of a tube within a sleeve. In this specification the sleeve is in two parts, one coaxially within the other and the passageways between chambers formed by resilient walls between the tube and sleeve are between the two sleeve parts. However, this device also suffers from the problem of tensile stresses, because any vibration must induce tensile loads in at least one of the resilient walls.